In recent years, advances in technology, as well as ever evolving tastes in style, have led to substantial changes in the design of automobiles. One of the changes involves the power usage and complexity of the various electrical systems within automobiles, particularly alternative fuel vehicles, such as hybrid, electric, and fuel cell vehicles. Many of the electrical components share a common direct current (DC) voltage bus. The DC voltage bus (alternatively referred to as a high voltage DC bus, a power bus, or an electrical bus) is configured to carry DC power from one or more DC power sources (e.g., a battery) in the vehicle to numerous other electrical components in the vehicle, such as auxiliary power modules, air compressors, power converters, and the like. In addition, a power inverter (or another suitably configured electric motor driver) is often connected to the DC bus and configured to drive the electric motor in the vehicle. The power inverter utilizes several semiconductor switches which are modulated (i.e., opened and closed) at a particular switching frequency to convert DC power from the bus to AC power for driving the electric motor. During operation, the power inverter generates a ripple current (or ripple voltage) at the switching frequency, which in some situations may not be entirely filtered by the power inverter (e.g., by an input capacitor and/or inductor). As a result, the ripple current is transmitted back out on the DC bus and potentially to the one or more of the other components coupled to the bus.
In general, each component coupled to the DC bus has an input filter comprising one or more input capacitors and/or inductors configured to provide specific input impedance which minimizes ripple current or voltage generated by the component. The input filter (e.g., the capacitors and/or inductors) of a respective component is selected to provide attenuation over frequency span suited for the operation of the respective component. The current rating of the input filter (or the ripple current rating of the component) is chosen such that it is not exceeded when the component is coupled to the bus during normal operation, without consideration for other components that may be coupled to the DC bus. This is because the other components are assumed to be configured to minimize the ripple they may generate, such that the total amount of ripple current on the DC bus is negligible.
However, because the input impedance of a particular component may vary with frequency, the component may have a reduced input impedance at a particular frequency. A current on the DC bus at (or near) this particular frequency results in an increased amount of ripple current that is dissipated by the particular component which, in turn, may result in a resonance at the particular frequency. When numerous electrical components, each having a different input filter frequency response, are coupled to the DC bus, the resulting electrical system often has one or more potential resonant frequencies.
In most systems, the power inverter utilizes variable switching frequency for purposes of optimizing one or more aspects of operation of the inverter, for example, to minimize switching losses or minimize torque ripple generated by the electric motor. However, when the switching frequency is changed, the frequency of the ripple current that may potentially be generated by the power inverter also changes. Depending on the switching frequency, the ripple current may produce a resonant current on the DC bus based on the quality factor and bandwidth associated with a particular resonant frequency. This resonant current may exceed the current rating of the input filter for one or more components coupled to the DC bus.
In some systems, the power inverter may be configured to avoid a particular switching frequency to prevent resonance from occurring on the DC bus during operation. However, the characteristics of the other components coupled to the DC bus may vary from vehicle to vehicle (e.g., as a result of individual component tolerances), and would therefore require customizing the power inverter configuration for each vehicle. In addition, the characteristics of the other components coupled to the DC bus may vary over time, for example, as a result of aging or a change of components. Therefore, it is difficult to ensure that the potential resonant frequencies are avoided during operation of the power inverter without repeatedly testing and/or diagnosing the system and configuring the power inverter before each time the vehicle is operated.